MY car's booster/master was painted hugger orange same as the car when I bought it. I've considered changing out the booster/ master or just painting it because it works great.
Any suggestions on painting it on the car and what type of preparation and paint would you use ? Sponge brush or spray ?
Thanks.

I would personally remove it, better access and not having to worry about masking everything up to avoid overspray. If you do paint, definitely go spray can, but depending on your budget, I would look into replating or getting a repro unit.

www.brakeboosters.com

Steve Gregori did mine, pics in my showroom.

I used a cast iron grey paint on mine. not correct but looks decent.

Usually,removing the master is the best way IMHO and experience.That way it makes it real easy to clean/sand/paint the booster.Block off the brake line inlets on the master so you don't get fluid all over. Paint the booster low gloss black and the master cast grey.Maybe buy some new nuts that hold the master on or at least wire wheel them.
Here is one I did recently:
http://www.chevelles.com/showroom/data/500/master1.jpg


The master cylinder was leaking,so it needed replacement anyway. The brake fluid made a nice mess of the silver paint on the booster :D
So it was just as easy to clean and paint the booster before I installed the new master.
Good enough for a driver in my book :)